Separately fired superheaters



Sept. '17, 1957 A. w. JACKSON 2,806,454

vSEPARATELX FIRED SUPERHEATERS Filed April 9, 1953 I 2 Sheets-Sheet lmay p 7, 1957 A. w. JACKSON 2,30 ,454

SEPARATELY FIRED SUPERHEATERS Filed April 9, 1953 2 Sheets-Sheet 2 T -4834 Y H United States Patent 2,80 6,f45 4- SEPARATELY? FIREDSUPERHEATERSArthur W, Jackson, Elkins' Park, Pan, assignbrt'dThe Babcock' 85' WilcoxC0mpany,:New"York, N. Y., a cor p'orationofNew Jersey Application April9, 1953, Serial Nth-347,749 6 Claims. (Cl122'4$5) This invention relatesin general to vapor heaters, and more particularly to separately fir'ed'convection heated steam superheaters.

Heretofore in separately fired steam superheaters' in which steam issuperheated to high temperatures by the heatingigases' generated bythe-'burning of fuel in'a re fractory walled furnace chamber,difficulties-have been encountered in providing highsuperheattemperatures Without refractory wall failures, tube slaggin'g where aslagging type of fuel is used, and'tube failures due to hightube-wall'ternperatures necessary. In the high temperature rangesencountered, the maintenanceof' tubes and furnace walls has been a majorproblem.- Slagging of' the tubes was particularly liable" tooccur whenthe steam heating tubes were arranged close together, but when the-tubeswere spaced far' enough" apartto avoid slag bridging,.theflow of heatinggases was usually notsuflieient to 'give the desired heating effect. Theexistence of excessively hot gas zones in'the comb'ustion' gas p'athofsuch units has frequently resulted in tube or wall failures at suchlocations.

In'some cases these difliculties have'be'en" reducedby the recirculationof' relatively cool flue gase's intoj the furnace'chamber, or theuse'of'high percentages 'ofi excess air'in the combustionof the fuel,'toattemp'erate tl'le -heat ing: gases generated: The effectiveness of gasreeireul'a: tiorr and excess air for this purpose dependsupon theirmanner of introduction and degreeof mixing 'withthe burning: fuel and/or fresh'heating gases.

The primary object of this i-nvention'is toprovidea separately firedvap'or heater with a pluralityi'of vapor heating passes in a simpleandcompact construction andarianged to provide optimum heatingv gastemperature and velocity'conditions for effective heat transfer'throughout-the'vapor heating passes without-danger of overheat ingthe vaporheater parts.

A further object is to provide a separately: firedvapor heater having animproved arrangement for recirculated gas-introduction andanunobstructed-gas mixingzpa'ss, to insure intimate mixingofthe-relatively coolreeirculated gases with the gaseous productsofcombustionbefore' reaching the vapor heating-surface.

A still-further object is to provide a separately fired vapor heaterwith vapor heatingelementsdispose'd in serially-connected paralleladjoining vapor heating-passes with a tube arrangement and vapor flowtherein to=provide optimum heating gas flow and heat-transferconditionsthroughout the passes.

The various features of novelty which characterize my. invention arepointed out-with particularity in the claimsannexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained=-by-'its-use,reference should be-hadtothe accompanying drawings and descriptivematterin which I have illustrated and-dc scribed several embodiments of myinvention.

Ofthe drawings;

Fig. lisa horizontal section, taken on the line 1-1 Z,8@5,4 54Patented'sept. 17, 1957 2. of Fig.- 2, of a separately fired steamsuperheater constructedin accordance with my invention; Fig.2 is asectional elevation taken on the line 2-2 of Fig. 1= through the firstsuperheater pass;

Figs 3 is a sectional elevation taken on the line 33 of Fig 1 throughthe third superheater pass;

Fig. 4 is a sectional elevation taken on the line 44 of Fig-.- 1 throughthe second superheater pass;

Fig. 5 is a transverse section taken on the line 5-5 of-Fig. 1;

Fig.6 is a view similar to Fig. 1, taken on the line 6-6 of Fig. 7 of asecond embodiment of myinvention; and

Fig.: 7 is a sectional elevation taken on the line 7-7 of Fig. 6. V

The separately fired steam superheater 1' illustrated in Figs. 1 -5comprises a rectangular refractory walled furnace orcombustion chamber 2defined by ,a front wall 3; rear wall 6, and side walls 7 and 8. A pairof fluid fuel burners 4,.which are enclosed by a windbox 33, arearranged to discharge through horizontally spaced burner ports421located intermediate the heightof the front wall 3: Theinner side wa1l'8isconstructed with a multiplicity of small-sized-reversely benthorizontal gas passages 11 arranged in vertical rows extendingthroughout the height of the side-wall adjacent the rear end thereof; Anarrow unobstructed gas mixing pass 12 is defined by the side wallSQthefront wall 3, the rear wall 6, and a refractory wall 43" arrangedparallel to and coextensive with the sidewall 8 The Wall 43 isconstructed with a. multi plicityof small sized reversely benthorizontal gas passages 13 similar to those in the'side' wall 8 butadjacent the front wall. I

The remaining space in the unit is occupied by a convectionvap orheating section containing three steam superheating: passes 14, 17, and20 arranged normal to the mixingpa'ss 12, this section being defined bythe wall 43,

extensions'of the front and rear walls 3 and 6, and arefractory'ontersi'de wall 42. The side wall 42 terminates short oftherearwall'6 to form a heating gas exit from trie'pass 20'. The threepasses 14, 17, and 20are'serially. connected adjoining parallel gaspasses defined by parallel vertical refractory walls 39 and 4tlextending'from'the' walls 43 an'd 42 respectively parallel to' thefrontand rearwall's 3an'd"6 Hollow tile 44 is'arranged along the bottomofthe'whole superheater to cool the floor.

Spaced groups" of multi-loop'ed nested pendent superheater tubes aredisposed in each of the steam superheating. passes 14, 17 and 20. Twobanks of tubes 15 and- 16 are-in the first pass 14', banks 19 and 18 inthe seeond pass 17, and banks-21, 22 and 23are in the third pass 20.-The supporting steel work for the unit com prises-roof beams 39,extending above a roof 41 and used to support the superheater tubes ineach gas pass. Hangers 45 carried'by. supporting cross-beams 46, 47, 48and 60 above the roof beams 39 are connected to the top intermediateloops of the superheater tubes in each pass. In the first gas passcorresponding tubes of the banks 15 and 16 are serially connectedbystraight horizontal tu bular elements 61. The other ends of the tubes inthe tube "banks 15 and 16 are connected to transverse inlet andintermediateheaders 36 and 37 respectively supported on the roof beams39. The header 36is joined by a connecting pipe 36a which leads to anoutlet header 36b ofth'e tube-bank 19 in the second pass 17. The otherend of'thi'stube bank 19 is connected to an intermediate header 35. Thetubes of the tube bank 18 in-the second pass are connected at oneend toan extension of the header 37. The other ends of the tubes of-bank 18are connected into a-transverse steam outlet header- 38. In the thirdpass 20 corresponding tubes in the banks 21, 22', and 23 are seriallyconnected by straight horizontal tubular elements 62 -and 63 ai1d extendbetween a steam inlet header 34 at the end of tube bank 23 and anextension of the header 35 at the end of tube bank 21. The upper loopedportions of the tubes and connecting ele ments of all tube banks extendinto the roof 41.

With the described construction and arrangement, the steam to besuperheated or reheated is introduced through the inlet header 34-,flows successively through the tube banks 23, 22 and 21 to theintermediate header 35, then through the tube bank 19 to header 36b,through the conduit 36a to header 36, through the tube banks 15 and 16to header 37, and then through the tube bank 18 to the superheated steamoutlet header 38 and discharge conduit 38a.

With the described arrangement and connection of the tube banks, thehottest heating gases entering the first pass 14 successively flow overthe tube banks 15, 16 and 18. These tube banks are arranged for steamflowin the same direction as gas flow, i. e. a parallel flow heattransfer relationship. The tubes of these banks are arranged four tubeswide on six inch centers, in contrast to the tubes of banks 19, 21, 22and 23 which are six tubes wide on four inch centers and have acounterflow heat transfer arrangement. With this arrangement the steammass flow in the tubes containing the higher temperature steam isgreater than the others due to the smaller number of tubes. This tendsto reduce the tube metal temperature and minimize the likelihood of tubefailure. Although the hotter heating gases contact the tubes in thefirst pass and a part of the second where the steam temperature ishighest, with the wider spacing of these tubes there will be a lower gasflow velocity and resulting heat transfer than where the tubes arecloser spaced in the remaining part of the second and third passes.However, in the third pass and part of the second pass where thecombustion gas temperature is lower, the tube banks are arranged to havethe steam enter at the end of the last pass and flow in a directionopposite the gas flow for counterflow heat transfer. Here the tubespacing is closer in order to provide more gas turbulence and higher gasvelocity with better heat transfer in this vapor heating section whereboth the steam and the heating gases are at their lower temperatures.

Combustion air is supplied to burners 4 from the windbox 33. The air isfirst preheated by forcing it through a rotary regenerative type heatexchanger 25 by a fan 31 and delivering the heated air through a duct 32to the windbox 33. The gaseous products of combustion from the furnace 2pass through the wall passages 11 into the mixing pass 12, horizontallythrough this pass and out through the wall passages 13. The flowrestrictions provided by these small wall passages and the narrow widthof the mixing pass cooperate in effecting a thorough mixing of the gasesbefore entering the convection heating section. The gases then flowthrough the narrow serially connected passages 14, 17 and 20 insuccessive intimate contact with the superheater tube banks 15, 16, 18,19, 21, 22 and 23, and exit through the gas outlet 24. The gases arewithdrawn from this outlet through a rotary regenerative type air heater25 by an induced draft fan 26 and discharged through a flue 27 to thestack 28. A damper 44 in the flue 27 controls the proportion of thegases discharged to the stack 28.

In accordance with my invention, relatively cool gases are withdrawnfrom the flue 27 and introduced into the furnace chamber 2 in a mannerproviding effective protection of the refractory Walls and permitting athorough mixing of the recirculated gases with the fresh gaseousproducts of combustion before the combined gases contact the steamsuperheating tubes. The quantity of gases recirculated is controlled bythe relative adjustment of the damper 44 and a damper 29 in arecirculated gas duct 36 connected into the flue 27 at the induced draftfan side of the damper 44. The duct 30 delivers recirculated gas to aninverted V-shaped manifold surrounding the top and sides of the burnerwindbox 33. The manifold 10 has a multiplicity of small ducts 9 ofsquare cross-section extending from the manifold 10 through thecombustion chamber wall 3 for the admission of recirculated gases. Withthe described flue arrangement the recirculated gases enter along theroof 41 and the side walls 7 and 8 of the furnace chamber so that thewalls and roof are protected from the high temperature gaseous productsof combustion generated by the burning fuel by a moving stream or layerof relatively cool recirculated gas which acts to absorb a substantialportion of the heat otherwise radiated to the furnace chamber walls. Athorough mixing of all of the gases is effected by the described mixingpass and the gas inlet and outlet openings thereto.

In the modification of my invention shown in Figs. 6 and 7, burners 50are arranged to discharge through the burner ports 50a in the outer sidewall 7, intermediate the height thereof and in a direction toward theopposite side Wall 8. With this burner arrangement, the recirculated gasmanifold 10 is arranged as previously described relative to the burnerports and the recirculated gases are introduced along the furnacechamber front wall 3, rear wall 6, and roof 41, through a multiplicityof ducts 57. Additional wall protection is provided by the use of avertical manifold 58 connected to the recirculated gas duct and manifolddischarge ducts 54 adjacent the chamber side wall 8 to provide streamsof cool flue gases along the wall 8.

In both embodiments gas recirculation is used to control the finaltemperature of the steam. By varying the quantity of gas recirculated,the quantity and temperature of the heating gases is varied with aresultant change in heat transferred to the steam.

In operation the gaseous products of combustion generated in thecombustion chamber 2 are in most cases at temperatures too high for safecontact with the steam superheating tubes. The radiant heat generatedfrom the combustion of the fuel in the combustion chamber is also toogreat to permit the direct exposure of the steam superheating tubes tothis radiant heat. To eliminate the possibility of tube metal failurethe steam superheating tubes are shielded from the furnace chamberradiation by the refractory walls 8 and 43. The recirculated flue gaseseffectively maintain the refractory furnace chamber walls at a safetemperature and the mixture of these CO'Oll gases with the hightemperature gaseous products of combustion by their passage through themixing pass insures a gas temperature at the superheater which willprovide the desired superheat tempenature, without overheating thetubes.

With the plurality of serially connected side-by-side steam heatingpasses of my invention, the refractory walls are less likely to beexposed to destructive temperature extremes, than would be so with asingle pass arrangement which would involve long refractory walls withgreat variations of temperature from one end to the other. The interiortransverse walls 39 and forming the heating passes have very littletemperature variation from one wall side to the other, since both sidesare exposed to the heating gases. The provision of three s-ide-by-sideheating passes provides an effective method of obtaining the desiredsuperheat temperatures with a minimum of connections between thesuperheater tube banks.

' While in accordance with the provisions of the statutes I haveillustrated and described herein the best forms of the invention nowknown to me, those skilled in the art will understand that changes maybe made in the form of the apparatus disclosed without departing fromthe spirit of the invention covered by the claims.

What is claimed is:

1. A separately fired high temperature vapor superheater comprisingrefractory walls defining a combustion chamber, means for burning fuelin suspension in said chamber, recirculated gas inlet means arranged todischarge gases along a vertical wall of said combustion chamber, anunobstructed gas pass of relatively narrow cross section arranged alongone side of said combustion chamber having restricted gas inlet andoutlet openings at opposite ends thereof for the flow of heating gasesfrom said combustion chamber, refractory walls defining a vapor heatingchamber having a plurality of serially connected adjoining parallel gaspasses arranged for the horizontal flow of gases in a direction normalto said .unobstructed gas pass and in communication with saidunobstructed gas pass outlet, a bank of vapor heating tubes disposed ineach of said vapor heating chamber gas passes, said vapor heating tubebanks being interconnected for vapor flow parallel to the direction ofgas fiow through an initial portion of the gas flow path through saidvapor heating passes and for counterflow during the remaining portion ofthe gas flow path through said vapor heating passes, and means forrecirculating a portion of the gases leaving said vapor heating chamberto said recirculated gas inlet means.

2. A separately fired high temperature vapor superheater comprisingrefractory walls defining a combustion chamber having a gas outlet inone vertical wall thereof, means for burning fuel in suspension in saidchamber, refractory exterior Walls and at least one vertical interiorwall defining a vapor heating chamber having a plurality of seriallyconnected adjoining parallel horizontally extending gas passes arrangedfor the horizontal flow of gases in a direction normal to the gas outletwall of said combustion chamber and in communication with saidcombustion chamber outlet, and one or more banks of vapor heating tubesdisposed in each of said vapor heating chamber gas passes, said vaporheating tube banks being interconnected for serial flow of vaporparallel to the direction of gas flow through an initial portion of thegas flow path through said vapor heating passes and for counterfiowduring the remaining portion of the gas flow path through said vaporheating passes.

3. A separately fired high temperature vapor superheater comprisingrefractory Walls defining a combustion chamber having a gas outlet inone vertical wall thereof, means for burning fuel in suspension in saidchamber, recirculated gas inlet means arranged to discharge gaseshorizontally along a vertical wall of said combustion chamber,refractory exterior walls and at least one vertical interior walldefining a vapor heating chamber having a plurality of seriallyconnected adjoining parallel horizontally extending gas passes arrangedfor the horizontal flow of gases in a direction normal to the gas outletwall of said combustion chamber and in communication with saidcombustion chamber outlet, a bank of vapor heating tubes disposed ineach of said vapor heating chamber gas passes, said vapor heating tubebanks being interconnected for serial flow of vapor parallel to thedirection of gas flow through an initial portion of the gas flow paththrough said vapor heating passes and for counterfiow during theremaining portion of the gas flow path through said vapor heatingpasses, and means for recirculating a portion of the gases leaving saidvapor heating chamber to said recirculated gas inlet means.

4. A separately fired high temperature vapor superheater comprisingrefractory walls defining a combustion chamber, means for burning fuelin suspension in said chamber, an unobstructed gas pass of relativelynarrow cross-section arranged along one side of said combustion chamberhaving restricted gas inlet and outlet openings at opposite ends thereoffor the flow of heating gases from said combustion chamber, refractoryexterior walls and at least one vertical interior wall defining a vaporheating chamber having a plurality of serial connected adjoiningparallel horizontally extending gas passes arranged for the horizontalflow of gases in a direction normal to said unobstructed gas pass and incommunication with said unobstructed gas pass outlet, and a bank ofvapor heating tubes disposed in each of said vapor heating chamber gaspasses, said vapor heating tube banks being interconnected for serialflow of vapor parallel to the direction of gas flow through an initialportion of the gas flow path through said vapor heating passes and forcounterflow during the remaining portion of the gas flow path throughsaid vapor heating passes.

5. A separately fired high temperature vapor superheater comprisingrefractory walls defining a combustion chamber having a gas outlet inone vertical wall thereof, means for burning fuel in suspension in saidchamber, refractory exterior walls and at least one vertical interiorwall defining a vapor heating chamber having a plurality of seriallyconnected adjoining parallel horizontally extending gas passes arrangedfor the horizontal flow of gases in a direction normal to the gas outletwall of said combustion chamber and in communication with saidcombustion chamber outlet, and a bank of vapor heating tubes disposed ineach of said vapor heating chamber gas passes, said vapor heating tubebanks being interconnected for serial flow of vapor parallel to thedirection of gas flow through an initial portion of the gas flow paththrough said vapor heating passes and for counterflow during theremaining portion of the gas flow path through said vapor heatingpasses, said tubes being positioned further apart in the initial portionof the gas flow path through said vapor heating passes than in thelatter portion.

6. A separately fired high temperature vapor superheater comprisingrefractory walls defining a combustion chamber, means for burning fuelin suspension in said chamber, a series of recirculated gas inletsarranged along one or more vertical walls of said combustion chamber,refractory walls defining an unobstructed gas pass arranged along oneside and in communication with said combustion chamber having curvedrestricted gas inlet and outlet openings at opposite ends thereof,refractory exterior walls and at least one interior vertical walldefining a vapor heating chamber having a plurality of seriallyconnected adjoining parallel horizontally extending gas passes arrangedfor the horizontal flow of gases in a direction normal to and incommunication with said unobstructed gas pass, one or more banks ofpendant-type nested multi-loop vapor heating tubes in each of said vaporheating chamber gas passes, said vapor heating tubes beinginterconnected for serial flow of vapor parallel to the direction of gasflow and also spaced further apart during an initial portion of the gasflow path through said vapor heating passes and for counterflow and withcloser tube spacing during the remaining portion of the gas flow paththrough said vapor heating passes, and means for recirculating a portionof the gases leaving said vapor heating chamber to said recirculated gasinlet means.

References Cited in the file of this patent UNITED STATES PATENTS1,064,250 Primrose et al June 10, 1913 1,102,361 Stevens July 7, 19141,691,698 Broido Nov. 13, 1928 1,891,879 Folliet et al. Dec. 20, 19321,929,890 Huet Oct. 10, 1933 FOREIGN PATENTS 970,916 France June 28,1950

